1. Field of the Invention
This invention relates to a light measuring circuit consisting of a photocurrent amplifying circuit which amplifies a current produced from a photoelectric conversion element and, more particularly, to a circuit which prevents the output of the photocurrent amplifying circuit from being saturated by a DC component.
2. Description of the Prior Art
Known photocurrent amplifying circuits includes a circuit which excels in linearity and responsivity and is capable of detecting a weak, relatively high frequency pulse-like light signal coming under an ambient light (external light) which extensively varies in a DC-like manner or at a low frequency (flicker). Unlike the photo-current amplifying circuits used for optical communication devices of pulse code modulation (PCM) systems, optical character readers, etc., the photocurrent amplifying circuit of this kind has been arranged, for example, as shown in FIG. 1 of the accompanying drawings.
The amplifying circuit shown in FIG. 1 has a negative feedback loop which includes a parallel connection circuit, formed by a T type low-pass filter circuit consisting of resistors R1 and R2 and a capacitor C having one terminal connected to a node between the resistors R1 and R2 and the other terminal grounded, and a gain limiting resistor R3 of a high resistance value disposed in between the output terminal 3 and the inverting input terminal 2 of an operational amplifier OP which has a high input impedance and has its non-inverting input terminal 1 grounded. A photoelectric conversion element PD (photogalvanic element), such as a photodiode which receives the light signal mentioned above, is connected between the input terminals 1 and 2 of the operational amplifier OP. The signal is taken out in the form of a photocurrent. The light signal is thus detected in a state of having good linearity over a wide range.
With the amplifying circuit arranged in this manner, assuming that the output current of the photoelectric conversion element PD, i.e. the photocurrent, is expressed as ip, the output voltage Vout of the amplifying circuit can be generally expressed as: ##EQU1##
In a low frequency region of .omega..apprxeq.0 in particular, the output voltage becomes: ##EQU2##
In a frequency region which is expressed as ##EQU3## it can be expressed as follows: EQU Vout=R3.multidot.ip (3)
As will be understood from Formula (2) above, in the low frequency region, the output of the amplifying circuit is determined by the sum (R1+R2) of the resistance values of the resistors R1 and R2. In a high frequency region, the amplifying circuit output is determined by the value of the resistor R3 as will be understood from Formula (3) above. Therefore, with the resistance value of the resistor R3 arranged to be a large value and the sum (R1+R2) of the resistance values of the resistors R1 and R2 arranged to be small, the signal component representing the ambient light can be suppressed while a signal component of a given frequency, representing a signal light to be handled, can be emphatically taken out. However, in the amplifying circuit arranged in this manner, the provision of T type low-pass filter circuit, consisting of the resistors R1 and R2 and the capacitor C in the negative feedback loop, causes a noise component generated at the operational amplifier OP to be also amplified. Compared with an arrangement having the resistor R3 solely included in the negative feedback loop of the operational amplifier OP, the noise component in the signal becomes larger. This results in a salient deterioration of the S/N ratio of the whole amplifying circuit.
Assuming that an operational amplifier having an FET input terminal is employed as the operational amplifier OP and that a current noise and a thermal noise are negligible, with a noise voltage which arises at the inverting input terminal 2 of the operational amplifier OP assumed to be en.sup.2, a noise voltage Von.sup.2 which arises at the output terminal 3 is expressed as follow: ##EQU4##
In the low frequency region of .omega..apprxeq.0, the noise voltage becomes as expressed below: EQU Von.sup.2 .apprxeq.en.sup.2 ( 5)
Further, in the high frequency region of .omega..apprxeq..infin., it becomes: ##EQU5##
In other words, in a given frequency region, the noise is increased by (1+(R3/R1)) times because of AC amplification as compared with an arrangement having the resistor R3 solely provided in the negative feedback loop of the operational amplifier OP.
This is further explained with reference to FIG. 2. FIG. 2 shows an AC circuit equivalent to the photocurrent amplifying circuit of FIG. 1. For an AC component, the impedance of the capacitor C is negligible. Therefore, the T type low-pass filter of FIG. 1 does not function as a filter and is considered equivalent to an arrangement having the resistor R1 connected between the inverting input terminal 2 of the operational amplifier OP and a grounding point as shown in FIG. 2. Further, the resistor R2 is equivalent to an arrangement having it connected as a load resistor between the output terminal of the amplifier OP and a grounding point. In FIG. 2, a reference symbol en denotes an equivalent voltage source reducing the input of noise generated by the operational amplifier OP by itself. However, in the case of a feedback arrangement, the non-inverting input terminal 1 and the inverting input terminal 2 have equal potentials. Therefore, with a bias current flowing to the inverting input terminal 2 of the operational amplifier OP not taken into consideration, the noise voltage en appears at the output terminal 3 of the operational amplifier OP in a state of having been amplified by the resistance value ratio between the resistors R1 and R3. It will be qualitatively understood that the noise component thus increases.
In order to prevent the output of the amplifying circuit from being satuarated by the DC photocurrent resulting from an ambient light with the conventional photocurrent amplifying circuit arranged as shown in FIG. 1, a DC removing ratio .gamma.=(R1+R2)/R3 should be arranged to be sufficiently small. However, the degree of noise amplification becomes too large for handling a weak signal. Therefore, there is a reducible limit to the DC removing ratio. Hence, the values of the resistors R1, R2 and R3 are generally determined according to a compromising point between a DC removing ration and a noise amplifying degree. As a result, it has been a drawback of the conventional arrangement that both the DC removing ratio and the noise amplifying degree become inadequate. To solve this problem, in cases where the above-stated amplifying circuit is to be used, it has been necessary to have another circuit or an optical system arranged to perform an additional function of compensating for the inadequacy of the DC removing ratio or the noise amplifying degree.